Direct conversion RF receivers are widely used. In such a receiver, the transmitted modulated RF signal is downconverted by mixing the RF signal with a local oscillator (LO) signal having about the same frequency as the carrier. The mixer subtracts the LO signal from the RF signal, leaving the baseband signal for further processing by the receiver. When two frequencies are mixed within a circuit generating some inherent distortion, or if there is some RF interference, intermodulation (IM) products exist. Some IM products can be filtered out and others are difficult to accurately filter out. In the case of a direct conversion RF receiver, a second-order intermodulation (IM2) component frequency is likely to be very close to, or within the bandwidth of, the baseband signal, making it difficult to filter out. Hence, it is desirable to provide a non-filtering circuit to remove such IM2 distortion.
Various methods have been used to remove such IM2 distortion in a differential demodulator. Such methods, however, are fairly complex and require significant silicon real estate to fabricate. The IM2 canceller of U.S. Pat. No. 8,000,676, for example, uses two feedback loops and requires two reference voltages to be generated to offset IM2 distortion of a common mode signal in a demodulator circuit of a direct conversion RF receiver.
What is needed is a simpler and smaller IM2 canceller for a differential demodulator.